Recently, not only is an electronics device becoming smaller, lighter and thinner, but also an electronic circuit is becoming high-speed processing. Therefore, it has been considerably required to achieve a miniaturization and a high frequency-wave performance of electronic components. For example, as to a portable electronics device such as a cellular phone, it is one of the most important challenges to achieve a smaller, lighter and thinner device (see Japanese Patent Kokai Publication No. 2003-163422, for example). Accordingly, it is needed that a miniaturization and a high frequency-wave performance are achieved by mounting various types of mounted-components with a short length of wiring at a high density.
In these circumstances, a flexible substrate which leads to achievement of a high-density mounting is getting much attention (see Japanese Patent Kokai Publication No. 2001-111189, for example). Hereinafter, a process for producing a conventional flexible substrate will be described with respect to FIGS. 1(a)-1(f).
First, as shown in FIG. 1(a), by use of a drill or laser machining, holes 502 for interlaminar connections are formed in an insulating sheet 501 being about 50 to 100 μm in thickness. Next, as shown in FIG. 1(b), the holes 502 are filled with a conductive paste 503 by performance of a printing method. Subsequently, as shown in FIG. 1(c), metal foils (i.e. copper foils) 504 are disposed on both surfaces of the insulating sheet 501, and thereafter the metal foils 504 are pressed in order to be stacked as shown in FIG. 1(d). After that, as shown in FIG. 1(e), resist films 505 having the same pattern as conductor circuits are formed on the metal foils 504. Subsequently, by using the resist films 505 as etch masks, a part of the metal foils 504 is etched away, followed by removing the resist films 505. As a result of that, a flexible substrate 500 having conductor circuits 506 is obtained as shown in FIG. 1(f).
Next, with respect to FIGS. 2(a)-2(c), a process for producing a conventional multilayer flexible substrate 550 will be hereinafter described.
First, as shown in FIG. 2(a), by using the flexible substrate 500 of FIG. 1(f) as a core substrate, insulating sheet 501a in which holes 502 thereof are filled with a conductive paste, as well as metal foils 504a, are stacked on that flexible substrate 500 in order to obtain a stacked substrate. Next, as shown in FIG. 2(b), resist films 505 are selectively formed on both surfaces of the stacked substrate. Subsequently, by using resist films 505 as etch masks, a part of the metal foils 504a is etched away, followed by removing the resist films 505. As a result of that, a multilayer flexible substrate 550 composed of four-layer conductor circuits is obtained as shown in FIG. 2(c).
As described above, an etching technique (i.e. a wet process) is employed for a purpose of forming the wire pattern in a case of a conventional production process. Therefore, an influence of an etchant on an insulating sheet is of concern. In this case, it was troublesome to perform washing and drying steps as a post-treatment. Furthermore, conventional wiring patterns formed by an etching technique are exposed to their surroundings on surfaces of a flexible substrate. This will cause a microcrack of the wiring patterns when the flexible substrate is folded, which will be far from satisfying in terms of a flexing life.
Considering that the conventional process comprises a step for forming through-holes and thereafter filling the through-holes with a conductive paste, such conventional production process is fundamentally the same as a process for producing a rigid substrate (i.e. typical print circuit). The above-mentioned step is cumbersome (because of taking about 3 hours), so it is desired to simplify or abbreviate it. However, it has been considered that such step is essential for producing a flexible substrate as well as a multilayer flexible substrate, and that it is therefore basically difficult to abbreviate it. Also, due to its consideration as an essential step, such step has been regarded as a matter of no concern. Therefore, there is no process for producing a flexible substrate and a multilayer flexible substrate with careful regard to the issues as described above.
Therefore, an object of the present invention is to provide a process for producing a flexible substrate and a multilayer flexible substrate wherein formation of through-holes and a filling of a conductive paste are abbreviated. Also, a further object of the present invention is to provide a flexible substrate and a multilayer flexible substrate as obtained by such process.